“Probably the most basic fact of desert geomorphology is that running water is by far the most important external agent of landform development .” — McKnight and Hess, p. 530.

1 The Topography of Arid Lands ❖ A Specialized Environment ❖ Running Water in Waterless Regions ❖ Characteristic Desert Surfaces—Ergs, Regs, and Hamadas ❖ The Work of the Wind ❖ Two Characteristic Desert Landform Assemblages in U.S. Deserts ❖ Summary

2 Wind Action Erosion By Wind Aeolian: wind processes; Aeolus was Greek god of winds Wind abrasion: particles carried by wind wear down a surface by impact

Deflation: lifting and transport in turbulent suspension by wind of loose particles of soil or regolith from dry ground surfaces

Deflation produces blowouts

Deflation and water action produce desert pavement ❖ The Work of Wind • Aeolian Erosion – Deflation Sand-blasted – Abrasion rock, or ventifact

Wind deflation

– Fig. 18-17 and 18-18 4 Wind Action Dust Storms Strong winds in arid and semiarid regions can lift large quantities of fine dust to produce dust storms Dust cloud may be several thousand meters high, and may represent an approaching cold front • Aeolian Transportation (Wind Transportation of Sediment) – Suspension – Saltation and Traction • Creep

– Fig. 18-19 6 • Reg—Stony Desert ‘Reg’ – Arabic for “stone” – Desert pavement • Hamada—Barren Bedrock ‘Hamada’ – Arabic for “rock” – Exposed bedrock – Cemented sediments – Fig. 18-16

7 – Rainfall – Fluvial deposition – Vegetation

– Fig. 18-4

8 ❖ Running Water in Waterless Regions • Significance of Running Water – Aeolian processes less significant – Sparse vegetation • Overland flow erosion • Surface Water in Deserts – Exotic Streams

– Fig. 18-5 9 ❖ A Specialized Environment • Special Conditions in Deserts – Weathering – Soil and regolith

-Fig. 18-1 10 • Fluvial Erosion in Arid Lands – Differential Erosion • Hardness of rock layers

Hard rock

Soft rock

– Fig. 18-1 11 – Fig. 18-9. The effects of differential erosion on the Red Cliffs near Gateway in Western Colorado.

12 – Ephemeral Streams – Desert Lakes • Playas • Playa lakes

– Fig. 18-6 and 18-7

13 – Desert Stream Channels

– Fig. 18-13. Mojave Desert, near Baker, CA. 14 – Fig. 18-8. Dry lake bed (playa) in central Nevada. 15 – Residual Erosional Surfaces • Inselbergs (“island mountains”) – Bornhardts

– Fig. 18-10. Kata Tjuta (the Oglas) in the desert of central Australia.

16 – Fig. 18-11. The development of a bornhardt.

17 • Pediments

– Fig. 18-12

18 – Piedmont Zone • Alluvial fan

– Fig. 18-29 19 • Fluvial Deposition in Arid Lands – Deposits in ephemeral stream channels – Piedmont area • Talus accumulations • Alluvial fans

– Fig. 18-14. Idealized cross section of a desert piedmont zone.

20 ❖ Two Characteristic Desert Landform Assemblages in U.S. Deserts

– Fig. 18-27 21 • Basin-and-Range Terrain – Ranges

– Fig. 18-28

22 Basin and Range Horst moves upward Graben moves downward

23 – Basin • Playa

– Fig. 18-30

24 – Basin • Salt pan

– Fig. 18-33. Death Valley, CA

25 • Mesa-and-Scarp Terrain – Horizontal layers of rock • Slopes have multiple scarps • Differential weathering

– Fig. 18-35 26 27 – Residual landforms • Flat-topped summits in horizontal sedimentary strata • Hard cap rock

– Fig. 18-36

28 Plateau Largest formation, with rivers cutting into it. Uplift is as fast as river cutting, which leads to Entrenched Meandering Rivers.

29 Mesa Smaller than a Plateau, larger than a Butte.

30 Mesa – Acoma Sky City A mesa is large enough for settlement. A butte is not.

31 Monument Valley

32 – Fig. 18-37. Mitten Buttes, Monument Valley, AZ.

33 34 Bryce National Park 35 Cave dwellings Softer layers erode, leaving behind large caves.

36 Balancing rock Differential weathering

37 – Arches and Natural Bridges • Minor erosional feature • Combination of weathering and fluvial erosion

– Fig. 18-39

38 Arches Differential weatheriing

39 ❖ Characteristic Desert Surfaces—Ergs, Regs, and Hamadas • Erg—A Sea of Sand – ‘Erg’ – Arabic for sand

– Fig. 18-15

40 – Badlands • Innumerable ravines and gullies. • Steep slopes.

– Fig. 18-38. 41 Arches National Park

42 – Dune Forms (Types) • Barchan • Transverse • Seif

– Fig. 18-21

43 – Soil Creep – Impermeable surface – Sand

- Fig. 18-2

44 • Aeolian Deposition Animation (Desert Sand Dunes) – Desert Sand Dunes • Vegetation and Dune Stabilization • Dune migration

– Fig. 18-20

45 Eolian Landforms Sand Dunes

Sand dune: hill or ridge of loose, well- sorted sand shaped by wind and usually capable of downwind motion • Active dunes constantly change form and move • Dunes may be stabilized by vegetation • Sand grains may bounce and move by saltation • Asymmetrical; downwind slip face maintains angle of repose of about 35º • Erg: sea of sand • Reg: desert pavement on top of vast sheets of sand – Barchan in Namibia

- Fig. 18-22

47 Sand Dunes

Barchan dune: isolated, crescent- shaped pile of sand; points of crescent face downwind

Transverse dune: wave- like ridges, transverse (perpendicular) to wind – Seif dunes in Simpson Desert of central Australia

- Fig. 18-23

49 Sand Dunes

Star dunes: large sand hills with dune crests radiating away from center

Parabolic dunes: crescent- shaped, points facing toward wind Coastal blowout dunes have saucer-shaped blowout in center • Major loess deposits of the world.

– Fig. 18-26 51 – Loess • Another aeolian feature that can be outside of deserts • Wind-blown silt, fine texture, no horizontal layering

– Fig. 18-25 52